1. Technical Field
This disclosure relates to a technology for conveying a recording sheet to a transfer position at an appropriate timing with an image formed on a moving image carrier.
2. Description of the Related Art
Many of image forming apparatuses, such as copiers and printers, that have become proliferate nowadays employ methods of: cyclically scanning (in a main scanning direction) a photoconductor drum that rotates (in a sub-scanning direction) with a laser beam being turned on and off according to image data, thereby forming a two-dimensional latent image on the photoconductor drum; developing a toner image, and transferring the image from the photoconductor drum or an intermediate transfer member, serving as an image carrier, onto a paper medium (recording sheet) at a transfer position. This method requires to appropriately convey the recording sheet (hereinafter, “sheet”) to the transfer position in timed relation to the image formed on the moving image carrier.
Such a sheet conveyance is generally performed using a feed roller that feeds a sheet from a tray, in which sheets are stored in a stacked manner, and a register roller provided immediately upstream of the transfer position. The related art will be described with reference to FIGS. 1 and 2 because the device configuration (hardware) for performing the sheet conveyance of the related art is identical with that of the present invention, which will be described later with reference to FIGS. 1 and 2.
With reference to FIG. 1, a toner image carried on a moving intermediate transfer belt 105 is transferred from the belt to a sheet 104 having been conveyed from a sheet feed tray 101 by a pair of secondary transfer rollers 107 and 116. First, only a sheet feed roller 102 is driven to deliver sheets stacked and stored in the sheet feed tray 101 one by one to a registration standby position. Simultaneously, a timer starts time measurement from a point in time where a registration sensor 120 for sensing arrival of the sheet senses a leading edge of the sheet. After a lapse of a predetermined period, the sheet feed roller 102 is stopped. The predetermined period of time set to the timer is a period of time after which a sheet is expected to be resiliently flexed into abutment against register rollers 103. The register rollers 103 are disposed immediately upstream of the secondary transfer rollers 107 and 116, and deliver the sheet at the registration standby position to the secondary transfer rollers 107 and 116 in accordance with a command to start registration issued in timed relation to movement of the toner image carried on the moving intermediate transfer belt 105.
The series of sheet conveyance operations is performed by controlling actuations of the sheet feed roller 102 and the register rollers 103. More specifically, the sheet conveyance operations are performed such that a central processing unit (CPU) 211 in a controller 210 of a control system shown in FIG. 2 executes a program for use in the operations for controlling a device group, e.g., the registration sensor 120, and a motor and clutches of a sheet conveyor 230.
FIG. 9 is a flowchart of a conventional sheet-conveyance control procedure performed in the prior-to-registration process.
As shown in FIG. 9, first, a sheet feed clutch 232 is turned on to cause the sheet feed roller 102 to convey the sheet 104 on the sheet feed tray 101 toward the register rollers 103 (step S1101).
Subsequently, the registration sensor 120 disposed on a transport path extending to the register rollers 103 senses a leading edge of the sheet 104 being conveyed (step S1102). Whether a preset period, after which the sheet is expected to be resiliently flexed into abutment against the register roller 103, has elapsed since the registration sensor 120 has sensed the leading edge of the sheet 104 is determined (step S1103).
When the preset period of time is determined to have elapsed (YES at step S1103), the sheet feed roller 102 is stopped. This state is referred to as a registration standby state, and maintained until a registration starting time.
Meanwhile, the registration starting time is in timed relation to arrival of the toner image carried on the intermediate transfer belt 105 at the secondary transfer roller 116. At the instant when the registration starting time has come (YES at step S1105), the sheet feed clutch 232 and a registration clutch 233 are turned on to cause the sheet feed roller 102 and the register rollers 103 to start rotation, thereby starting registration (step S1106).
After registration is started, processing moves to a subsequent process in which the secondary transfer roller 116 transfers the toner image onto the sheet, and thereafter to a process in which a fixing unit 122 fixes the toner image onto the sheet.
However, even when a command for causing the sheet feed roller 102 and the register roller 103 to start rotation simultaneously is issued in the prior-to-registration process as in the conventional control procedure shown in FIG. 9, the sheet feed roller 102 and the register rollers 103 do not necessarily start rotation at the same time due to fluctuations in engagement duration of the sheet feed clutch 232 and that of the registration clutch 233, and the like.
When the register rollers 103 start rotation earlier than the sheet feed roller 102, the sheet must be conveyed by a conveying force exerted only by the register rollers 103. When the degree of resilient flexure of the sheet is small in this state, the sheet becomes less flexible and receives a load from the sheet feed roller 102, which results in skid of the sheet. Accordingly, timing between arrival of the toner image carried on the intermediate transfer belt 105 at the secondary transfer roller 116 and sheet feeding is desynchronized. This result in misregistration of the leading edge of an image formed on the sheet.
Conventionally, sheet-feed control methods that control sheet conveyance operations according to a pre-determined appropriate degree of resilient flexure have been proposed (see Japanese Patent Application Laid-open No. H6-56302). According to the method described in Japanese Patent Application Laid-open No. H6-56302, the sheet conveyance operation to a registration position is feedback controlled. More specifically, the degree of resilient flexure is estimated based on a previously detected travel (number of pulses) of a sheet having been conveyed, and the sheet conveyance operation is feedback controlled so that sheet undergoes an appropriate degree of resilient flexure.
Hence, the method disclosed in Japanese Patent Application Laid-open No. H6-56302 that allows appropriate conveyance of a sheet is not premised on such a sheet that is insufficiently flexed at the registration standby position. Furthermore, the method disclosed in Japanese Patent Application Laid-open No. H6-56302 requires a sensor for detecting a rear end of a sheet in addition to the registration sensor and calculations for estimating the degree of resilient flexure based on a conveyance travel detected with at least the two sensors and a target value. These requirements serve as constraints.